Generally stated, radiotelephones receive and transmit radio-frequency ("RF") radiotelephone signals via incoming and outgoing receive and transmit signal paths. The receive and transmit signal paths typically include components such as microphones and antennas operably associated with a printed circuit board (for signal processing and other functions) disposed in a radiotelephone housing.
Typically, the microphone is positioned in the radiotelephone such that during use it is close to a user's mouth and free to receive the auditory signals associated with a user's speech. Generally described, the microphone receives the auditory signals corresponding to a user's speech and then relays the received auditory signals to the printed circuit board for further processing. The received signal is ultimately directed as an output signal out of the radiotelephone to a remote site. The output signal is typically transmitted out of the radiotelephone through a signal path which includes the antenna.
Many of the more popular hand-held telephones are undergoing miniaturization. Indeed, many of the contemporary models are only 10-12 centimeters in length. Because the printed circuit board is disposed inside the radiotelephone, its size is also shrinking, corresponding to the miniaturization of the portable radiotelephone. As the radiotelephone reduces in size, it constrains the physical configuration or arrangement of the internal components. For example, many integrated components such as the antenna and microphone can be positioned in relatively close proximity of each other. Unfortunately and undesirably, RF energy from the antenna can find its way into the receiving signal path and distort the sound as received in the microphone. Indeed, the distortion is generally frequency dependent and produces an audible (and potentially irritating) sound which is descriptively termed "motorboating".
Generally stated, microphones are difficult to adequately shield due to design constraints such as acoustic isolation, short conductor leads for low signal loss, and an aperture for sound transmission disposed adjacent the aperture in the housing as described above. An electrical connection is made between the microphone and the printed circuit board through flexible means such as wires, elastomeric connectors, and springs. Some of the connections, such as the elastomeric connector, can be sandwiched between the microphone and the printed circuit board to dampen or absorb shocks and help acoustically isolate and thus protect the integrity of the signal at this connection.
In the past, some radiotelephones have metallized the housing front next to the microphone or next to the perimeter of the rubber mounting connector, these have had limited success in attempting to resolve the motorboating condition. Further disadvantageous, metallizing components can be costly. Additionally, capacitors have been used in an attempt to filter the signal distortions due to RF interference. Unfortunately, these attempts have had less than satisfactory results. This is especially true as the operating frequencies of the radiotelephones increase. Indeed, many radiotelephones can operate at frequencies at 800 Mhz and greater, even up to 1.9 GHz. Unfortunately, the signal distortions can increase or become more problematic as the operating frequency increases. Further, many radiotelephones operate in dual band or full duplex; each of which can potentially further exacerbate the problem. For example, the dual band antenna's radiation pattern may produce additional RF energy directed at the microphone.